The present invention relates to an image sensor; and, more particularly, to a color image sensor and a method for fabricating the same.
Generally, an image sensor, e.g., a CCD (charge coupled device) image sensor or a CMOS (complementary metal oxide semiconductor) image sensor is employed to scan and convert an optical image into electrical signals.
Referring to FIG. 1, there is shown a pixel unit of a conventional image sensor including a photodiode (PD) and four NMOS transistors. The four NMOS transistors include a transfer transistor 12, a reset transistor 14, a drive transistor 16 and a select transistor 18. The transfer transistor 12 transfers photoelectric charges generated in the photodiode to a sensing node. The reset transistor 14 resets the sensing node in order to sense a next signal. The drive transistor 16 acts as a source follower. The select transistor 18 outputs data to an output terminal OUT in response to an address signal. A load transistor 20, which is positioned between the output terminal OUT of the pixel unit and the ground voltage level, receives a biasing signal from an external device for the sake of biasing the pixel unit. A capacitance of a floating diffusion is referred to as xe2x80x9cCfdxe2x80x9d.
Referring to FIG. 2, there is shown the photodiode of the pixel unit in FIG. 1. The photodiode includes a P+ silicon substrate 21, a P-epi (epitaxial) layer 22, field oxide layers 23, an Nxe2x88x92 diffusion region 24 and a P0 diffusion region 25. The photodiode (PD) has a PNP junction structure in which the P-epi 22, the Nxe2x88x92 diffusion region 24 and the P0 diffusion region 25 are stacked.
Referring to FIG. 3, a conventional color image sensor includes pixel units and each pixel unit includes a photodiode. Photodiodes 102a, 102b and 102c are formed in the color image sensor. Non-photosensing regions 103 are formed between photodiodes 102a, 102b and 102c. An interlayer insulating layer 105a is formed on the photodiodes 102a, 102b and 102c and the non-photosensing regions 103. Light shield layers 104 is formed on the interlayer insulating layer 105a to shield light incident on the non-photosensing regions 103. An interlayer insulating layer 105b is formed on the light shield layers 104. Red, green and blue filters 106a, 106b and 106c are formed on the interlayer insulating layer 105b. A buffer layer 107 is formed on the color filters 106a, 106b and 106c to improve the planarization of upper portions of the color filters 106a, 106b and 106c and the transitivity of light. Micro lenses 108a, 108b and 108c are formed on the buffer layer 107.
The photodiode 102a receives light from an object and integrates photoelectric charges from the light via the micro lens 108a and the red filter 106a. The photodiode 102b receives the light of the object and integrates the photoelectric charges from the light via the micro lens 108b and the green filter 106b. The photodiode 102c receives the light of the object and integrates the photoelectric charges from the light through the micro lens 108c and the blue filter 106c. The pixel units consist of red, green and blue pixel units to output color signals of red, green and blue components of the light from the object. Conventionally, the red, green and blue pixel units are formed by the same fabrication process, respectively, so that the photodiodes 102a, 102b and 102c of the red, green and blue pixel units have substantially the same depletion region structure each other. Accordingly, there is a problem that the conventional color image sensor having photodiodes of the same depletion region structure may not improve light sensitivity since it is variable according to wavelengths of the red, green and blue components of the light.
Referring to FIG. 4, a graph depicts the absorption length of a light wavelength in Si, GaAs and Ge mediums when light is incident on a medium. The light wavelength of 770 nm is absorbed up to the absorption length of 7 xcexcm in the Si medium while the light wavelength of 390 nm is absorbed up to the absorption length of 0.1 xcexcm in the Si medium. Therefore, the light sensitivity of a photodiode depends on a kind of medium and wavelengths of red, green and blue components of light.
Referring to FIG. 5, a graph depicts light sensing characteristics of the conventional color image sensor in FIG. 3. The photodiode of the color image sensor receives light from an object. The light includes red, green and blue components, which have a different wavelength from each other. The red component has the longest wavelength among the red, green and blue components. The green component has a wavelength longer than the blue component. Since the blue component has a wavelength shorter than the red or green component, a part of the blue component is lost before the blue component reaches the depletion region. Accordingly, the photodiode have low light sensitivity of the blue component. Also, when a red pixel unit reaches a saturation state, a blue pixel unit does not reach the saturation state. Furthermore, when an image process is performed on the basis of color signals from the red pixel unit, color signals from the green and blue pixel units can be lost, so that colors represented by the conventional image sensor are limited and a dynamic range for the sake of processing data is reduced. In other words, color signals from the red, green and blue pixel units may display different colors from actual colors of an object because the red, green and blue pixel units have different light sensing characteristics from each other.
It is, therefore, an object of the present invention to provide a color image sensor in which a plurality of photodiodes have a different depletion region structure from each other to improve light sensitivity for various color components of light from an object.
It is another object of the present invention to provide a method for fabricating a color image sensor in which a plurality of photodiodes have a different depletion region structure from each other to improve light sensitivity for various color components of light from an object.
In accordance with an aspect of the present invention, there is provided a color image sensor for scanning and converting an optical image into electrical signals, comprising: a red photodiode including a P-type semiconductor layer and a first N-type diffusion region located beneath the surface of the P-type semiconductor layer to thereby form a first depletion region positioned beneath the surface of the P-type semiconductor layer; a green photodiode including the P-type semiconductor layer and a second N-type diffusion region located beneath the surface of the P-type semiconductor layer to thereby form a second depletion region positioned beneath the surface of the P-type semiconductor layer, wherein the second depletion region is more adjacent to the surface of the P-type semiconductor layer than the first depletion region; and a blue photodiode including the P-type semiconductor layer and a third N-type diffusion region located beneath the surface of the P-type semiconductor layer to thereby form a third depletion region positioned beneath the surface of the P-type semiconductor layer, wherein the third depletion region is more adjacent to the surface of the P-type semiconductor layer than the second depletion region.
In accordance with another aspect of the present invention, there is provided a method for fabricating a color image sensor for scanning and converting an optical image into electrical signals, comprising the steps of: (a) forming a P-type semiconductor layer on a substrate; (b) forming field oxide layers on the P-type semiconductor layer to define regions for red, green and blue photodiodes; (c) providing an ion implantation mask having different mask patterns for the red, the green and the blue photodiodes; (d) implanting impurity ions into the P-type semiconductor layer through the use of said ion implantation mask to form N-type diffusion regions in the P-type semiconductor layer; and (e) applying a thermal process to the resulting structure to form different first, second and third depletion regions corresponding to the red, the green and the blue photodiodes.